Class D amplifier no low pass filter feedback with zero phase delay

• US 5,949,282 A
• Filed: 02/25/1998
• Issued: 09/07/1999
• Est. Priority Date: 02/25/1998
• Status: Expired due to Term

• Current Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Original Assignee: National Semiconductor Corporation (Texas Instruments, Inc.)
• Inventors: Takagishi, Hideto, Nguyen, Huey
• Primary Examiner(s): Pascal, Robert
• Assistant Examiner(s): Choe, Henry
• Application Number:
  US09/030,746
• Time in Patent Office: 559 Days
• Field of Search: 330/10, 330/207 A, 330/251, 330/85
• US Class Current: 330/10
• CPC Class Codes: H03F 3/2173 of the bridge type

Abstract

A circuit for generating an accurate reproduction of the output of a Class D amplifier for error-correction purposes includes a reference signal amplifier circuit. The reference signal amplifier circuit includes an op-amp configured as a differential amplifier that combines the digital outputs of the internal output stage of the amplifier into a single reference signal. By reducing the bandwidth of the op-amp, switching noise is removed from the digital signals, without adding any phase shift to the generated reference signal. In addition, by using the digital signals directly from the output stage rather than the filtered analog outputs of the amplifier, distortion due to external speaker-induced back-EMF is prevented. The reference signal can then be compared to the original signal input to the amplifier for error-correcting purposes.

Claims

1. A Class D amplifier, comprising:

a pulse-width modulator for converting an input audio signal into a digital waveform;

an output stage for applying signal splitting and level shifting to said digital waveform to produce high and low digital driving signals; and

an error amplifier circuit for comparing the output signal of said Class D amplifier to said input audio signal for error-correction purposes;

a reference signal amplifier circuit directly coupled to said output stage for filtering noise from said high and low digital driving signals, and combining said high and low digital driving signals into a single analog reference signal, said reference signal amplifier circuit having a roll-off frequency greater than the frequencies of said high and low digital driving signals.

2. The circuit of claim 1 comprising a low-bandwidth op-amp configured as a differential amplifier.

3. The circuit of claim 2 wherein said low-bandwidth op-amp has a rolloff frequency substantially less than the frequency of switching noise generated by said output stage.

4. In a Class D amplifier having a pulse-width modulator for converting an incoming audio signal into a digital waveform, and an output stage for applying signal-splitting and level-shifting to said digital waveform to produce high and low digital driving signals, a method, comprising the steps of:

reading said high and low digital driving signals directly from said output stage;

filtering out switching noise from said high and low digital driving signals; and

combining said high and low digital driving signals into a single reference signal using a reference signal amplifier circuit having a roll-off frequency greater than the frequencies of said high and low digital driving signals.

5. The method of claim 4 wherein said filtering, averaging and combining steps are performed by a low-bandwith op-amp configured as a differential amplifier.

6. The method of claim 5 wherein said low-bandwidth op-amp has a rolloff frequency substantially less than the frequency of said switching noise.